Due to their advantageous low power consumption and lack of toxic materials, solid state light emitting diode (LED) lighting applications are rapidly replacing conventional incandescent and florescent lighting systems. However, an LED cannot be exposed to the AC mains like an incandescent bulb. Solid state lighting applications thus include a switching power converter to convert the AC input voltage into a rectified output voltage that may power the LED. It is also conventional for incandescent light bulbs to be powered through a dimmer switch that includes a triac. The triac requires a minimum level of current known as the holding current for its operation. In an incandescent application, the bulb directly connects to the dimmer such that there is no issue with regard to supporting the triac holding current.
Since the users expect an LED to mimic the behavior of incandescent bulbs, the corresponding switching power converter needs to support the required holding current. It is thus conventional for solid state lighting (SSL) switching power supplies to include a bleeder circuit that is active during periods of applied dimming through the dimming switch. The bleeder circuit conducts sufficient current despite the application of dimming so that the triac in the dimmer for the SSL switching power converter conducts its required holding current.
To supply the internal power supply voltage, conventional SSL switching power converters may include a switch that couples a storage capacitor through a resistance to a charged node. Since the internal power supply voltage is commonly denoted as VCC, the corresponding switch is designated herein as the VCC switch to distinguish it from the power switch that the switching power converter cycles to regulate the output voltage. The VCC voltage drops when the VCC switch is cycled off by a product of the charging current drawn by and an equivalent series resistance (ESR) for the storage capacitor. If the capacitor ESR and charging current product is large, the VCC voltage may drop below a minimum required value such that the power converter performs a power on reset (POR). The storage capacitor may then be charged back, whereupon the VCC switch is cycled off and the VCC voltage again drops below the minimum required value. The result is that the LED bulb powered by the corresponding SSL switching power converter is unable to properly startup.
Accordingly, there is a need in the art for a switching power converter with an improved regulation of the internal power supply voltage.